Topic 4: Chemical Change

• What is oxidation/reduction?


  Oxidation - When a substance gains oxygen or loses electrons

  Reduction - When a substance loses oxygen or gains electrons
• What is the reactivity series of metals? What are the trends in the reactivities of metals in reactions with acids/water?


  The series shows the metals in order of their reactivity.

  Metals above H₂ in reactivity series react with acid to produce H₂. The more reactive the metal is, the quicker and more violent reaction with acid occurs.

  Metals below H₂ don't react with acids.

  Not all metals above H₂ react with water - mostly Group 1 and 2 metals. Aluminium is the borderline case.
• What is a displacement reaction?

  A reaction where a more reactive metal displaces a less reactive metal from a compound.
• How are unreactive metals found in Earth?

  In their natural state (well, they are unreactive)
• How can metals less reactive than carbon be extracted?

  Reduction with carbon. Carbon displaces the metal in a metal oxide - gets oxidised to carbon oxides. Metal from the metal oxide gets reduced to the pure metal.
• How are metals more reactive than carbon extracted?

  By electrolysis
• What is the general equation for a reaction between metals and acids? what type of reaction is this?


  Metal + Acid → Salt + Hydrogen gas.

  This is a single displacement reaction, redox reaction
• Which metals in the reactivity series will react with acid?

  Those above hydrogen.
• What is the general equation for a neutralisation reaction?

  base + acid = salt + water
• what is the general equation for the reaction between metal carbonate and acid?

  Metal carbonate + acid → salt + water + carbon dioxide
• What is the general equation for the reactions between metal oxides and acids?

  Metal oxide + acid = a salt + water
• What is a redox reaction?

  A reaction where both oxidation and reduction occurs
• Explain in terms of gain or loss of electrons which species has been oxidised and which species has been reduced when magnesium reactions with hydrochloric acid


  Magnesium has lost electrons and thus has been oxidised ( Mg to Mg2+)

  The hydrogen in HCl has gained electrons and thus has been reduced (H+ to H₂)
• How is a soluble salt formed?

  1. React the excess acid with some insoluble chemical
  2. Filter off the leftovers
  3. Crystallise the product
• What do acids and alkalis produce in aqueous solutions?

  Acids produce hydrogen ions, alkalis produce hydroxide ions
• What are bases, acids and alkalis?

  Bases are compounds that neutralise acids, acids produce hydrogen ions in aqueous solutions, alkalis are soluble bases - produce hydroxide ions in aqueous solutions.
• What is the pH scale and what does a pH of 7 show?

  pH scale: measures acidity/alkalinity. pH of 7: neutral.
• State the general equation for a neutralisation reaction in a short, ionic form.

  H+ + OH− → H2O
• What is a strong acid and weak acid?

  Strong acid is completely ionised in aqueous solution; weak acid is only partially ionised in aqueous solution.
• What happens to pH as the concentration of H+ increases?

  The pH decreases
• what is a concentrated acid and what is a diluted acid? Is this the same as a strong and weak acid?

  Concentrated acid has more moles of acid per unit volume than dilute (dilute refers to solutions of low concentrations)

  It is not the same - concentration is not the same thing as strength of an acid

  Strength refers to whether the acid id completely ionised in water (strong) or only partially (weak)
• As the pH decreased by one unit, what change is seen in the hydrogen ion concentration?

  Increases by a factor of 10
• What is electrolysis?

  The passing of an electric current through ionic substances that are molten or in solution to break them down into elements; ions are discharged (they lose/gain electrons) at electrodes to produce these.
• What is an electrolyte?

  The liquid/solution which conducts electricity
• What is a cathode and what is an anode?

  Cathode is the negative electrode, anode is the positive electrode
• What occurs at the cathode and what occurs at the anode during electrolysis?

  Reduction occurs at the cathode

  Oxidation occurs at the anode
• In aqueous electrolysis, which element is discharged at the cathode? Oxygen is produced at the anode unless what?

  The less reactive element discharges at the cathode. Hydrogen is produced unless there is a less reactive metal, in which case the said metal is produced. Oxygen is produced at the anode unless the solution contains halide ions, in which case halogen molecules are produced.
• How is aluminum manufactured? Why is it expensive?

  Aluminium is made through the electrolysis of aluminum oxide and cryolite. Lots of energy is needed to produce the current in electrolysis which makes this process expensive.
• Why is cryolite used in the manufacturing of aluminum?

  It lowers the melting point of aluminium oxide, reducing energy costs.
• What are the half equations in the extraction of aluminum?

  Al 3+ + 3e- → Al (cathode)

  2O2- → O2 + 4e- (anode)

  Oxygen reacts with C of the anode producing Co2
• What are the half equations in electrolysis of the aqueous KCl


  K+  +  e-  =     K (cathode)2Cl-  =  Cl2 + 2e- (anode)
• Titration practical


  1. Use the pipette to add 25 cm3 of alkali to a clean conical flask

  2. Add a few drops of indicator and put the conical flask on a white tile

  3. Fill the burette with acid and note the starting volume

  4. Slowly add the acid from the burette to the alkali in the conical flask, swirling to mix

  5. Stop adding the acid when the end-point is reached (this is when the acid has neutralised the alkali and the indicator changes colour)

  6. Note the final volume reading, and calculate how much acid you added in total

  7. Repeat the titration until you get 'concordant results', which means volumes of acid that are within 0.10 cm3 of each other

  8. Use the concordant results to calculate the mean volume of acid required to neutralise the alkali
• Why should you swirl the conical flask during the titration?

  To evenly distribute the added acid/alkali
• Methyl orange in red in acidic solutions and yellow in alkaline solutions 
• When acid molecules are added to water and split apart, we say that they 'ionise' or 'dissociate'. These two words both mean the same thing. 
• Strong acids ionise completely, whereas weak acids only partially ionise. 
• What describes the strength of an acid?

  The proportion of acid molecules which dissociate into hydrogen ions
• Acid + metal oxide ➔ salt + water
• Acid + metal hydroxide ➔ salt + water
• How to obtain soluble salt crystals from an acid-base reaction

  PART 1

  Place dilute acid in a beaker and heat gently

  Add the solid base bit by bit until it stops reacting, which means its in excess

  Isolate the salt solution by filtering out the excess solid base using filter paper and a funnel.